Field and Laboratory Examination of Uranium Microcrystallization and Its Role in Uranium Transport
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Field and Laboratory Examination of Uranium Microcrystallization and Its Role in Uranium Transport Takashi Murakami1, Toshihiko Ohnuki2, Hiroshi Isobe3 and Tsutomu Sato4 1 Dept. of Earth and Planetary Science, Univ. of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan (e-mail: [email protected]), 2 Dept. of Environmental Science, Japan Atomic Energy Res. Inst., Tokai 319-1106, Japan, 3 Dept. of Earth Sciences, Kumamoto Univ., Kumamoto 860-8555, Japan, 4 Div. of Global Environmental Science and Engineering, Kanazawa Univ., Kanazawa 920-1192, Japan ABSTRACT Adsorption is believed to be a dominant mechanism of uranium distribution between solid and solution, and thus, to play a major role in uranium transport. Because iron oxides and hydroxides are abundant at the Earth’s surface and are great adsorbents of uranium, we have examined natural rocks that contain iron minerals along with uranium, and also carried out Fe-U coprecipitation and aging experiments to find how uranium is distributed between Fe minerals. Transmission and scanning electron microscopy reveals that microcrystals (10-50 nm) of metatorbernite (Cu(UO2)2(PO4)2•8H2O) are scattered within nodules consisting of fine-grained (2-50 nm) goethite and hematite, where the ground water is undersaturated with respect to metatorbernite, for the natural rocks from the Koongarra ore deposit, Australia. The microscopy also reveals that microcrystals (a few nm) of dehydrated schoepite ((UO2)O0.25(OH)1.5) are formed among fine-grained hematite after aging coprecipitated Fe-U in the laboratory, and the solution is undersaturated with respect to schoepite. The beam size of microscopes is found to be important for the chemical analysis of such microcrystals. We detect a strong signal of uranium for a beam size < 40 nm; whereas a weak uranium signal is obtained for a beam size > 150 nm. Our results indicate that such a weak uranium signal should not be taken as a result of homogeneously distributed uranium over goethite and hematite surfaces by, for instance, adsorption. The micrcrystallization observed in both the field and laboratory suggests that fine grained uranyl minerals play a major role in uranium transport and migration. INTRODUCTION The long-term safety of high-level waste disposal sites could be assessed by laboratory experiment and simulations. However, it cannot be convincing if the assessment is only based on experimental and calculated results because of the long half-lives of the actinide elements present in nuclear wastes. It is necessary to understand the long-term transport of these elements. However, direct understanding of the transport is quite difficult because the actinide elements such as Pu, Np, and Am in natural environments are too low in concentration to measure or estimate their transport. The long-term uranium transport provides deeper understandings for long-term migration of actinides [1]. Uranium(4+) is insoluble under a reducing condition, forming mainly uraninite (UO2+x). On the other hand, U(6+) is much more mobile due to a high solubi
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